This application is based on Patent Application No. 2001-31483 filed Feb. 7, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical demultiplexer and an optical multiplexer having optical multiplexer or demultiplexer elements of a multi-stage Mach-Zehnder configuration connected together in tandem and used in fields such as optical communication, photonic switching, and optical computing.
2. Description of the Related Art
In recent years, an optical multiplexer and an optical demultiplexer have been more and more important in the fields of optical communication, photonic switching, and optical computing and notably in the field of wavelength multiplexing optical communication in which optical signals of different wavelengths are multiplexed for communication; the optical multiplexer multiplexing two signal lights of different wavelengths and outputting the multiplexed light from one output port, the optical demultiplexer demultiplexing a light having its wavelength multiplexed and outputs the demultiplexed lights from two output ports. Further, an interleave filter in which a passband and a stopband have an equal bandwidth is gathering much attention as a part that is combined with an AWG (Arrayed Waveguide Grating) to double the number of AWG channels.
As a part that meets these requirements, for example, M. Oguma, et al., xe2x80x9cFlat-passband interleave filter with 200 GHz channel spacing based on planar lightwave circuit-type lattice structurexe2x80x9d, Electronics Letter, 2000, Vol. 36, no. 15, pp. 1299-1300 describes an optical multiplexer or demultiplexer elements in which the passband and stopband have flat characteristics. Further, T. Chiba, et al., xe2x80x9cWavelength Splitters for DWDM Systemsxe2x80x9d, LEOS Topical Meeting 2001, MD2.2, pp. 11-12 reports a composite optical multiplexer or demultiplexer having optical multiplexer or demultiplexer elements connected together in tandem in two stages because a single optical multiplexer or demultiplexer element does not provide a sufficient stop value.
FIG. 9 shows the circuit configuration of a conventional optical demultiplexer. The demultiplexer has two-stage optical demultiplexer elements 90-1, 90-2, and 90-3 connected together in tandem in two stages and having optical path length differences of 1:xe2x88x922, 1:xe2x88x922, and xe2x88x921:2, respectively. The optical path length of difference is normalized a unit path length difference xcex94L. The positive optical path length difference is defined to mean that in the two arms of each Mach-Zehnder circuit, the upper arm has a larger optical path length than the lower arm. On the other hand, the negative optical path length difference is defined to mean that the lower arm has a larger optical path length than the upper arm. In the 1st stage, the one two-stage optical demultiplexer element 90-1 (corresponding to N=2) is disposed, and in the 2nd stage, the two two-stage optical demultiplexer elements 90-2 and 90-3 are disposed.
The two-stage optical demultiplexer elements 90-1, 90-2, and 90-3 are constructed in a two-stage Mach-Zehnder form using three directional couplers 93-1 to 93-9 for each element. Phase shifters 94-1 to 94-6 are installed on each optical path to control phase.
With a tandem-connected optical demultiplexer, when a wavelength multiplex signal of an equi-channel spacing xcex1, xcex2, . . . , xcexM (M is an integer equal to or larger than 2) is input from an input port IN1a of the two-stage optical demultiplexer element 90-1 in the 1st stage, a wavelength multiplex signal of xcex1, xcex3, . . . , xcexMxe2x88x921 is output from one selected output port OUT2c of the two-stage optical demultiplexer element 90-2 in the 2nd stage, while a wavelength multiplex signal of xcex2, xcex4, . . . , xcexM is output from one selected output port OUT4c of the two-stage optical demultiplexer element 90-3 in the 2nd stage.
With such a tandem-connected optical demultiplexer, the output ports can be selected in 2xc3x972=4 ways depending on which of the two output ports OUT1c and OUT2c of the two-stage optical demultiplexer element 90-2 in the 2nd stage is selected and on which of the two output ports OUT3c and OUT4c of the two-stage optical demultiplexer element 90-3 in the 2nd stage is selected.
The through output port (that is, the output port having its optical waveguide physically coupled to the corresponding input port) OUT2c of the two-stage optical demultiplexer element 90-2 is selected, the optical demultiplexer element 90-2 being connected to the through output port OUT1b of the two-stage optical demultiplexer element 90-1 in the 1st stage. The cross output port (that is, the output port not having its optical waveguide physically coupled to the corresponding input port) OUT4c of the two-stage optical demultiplexer element 90-3 is selected, the optical demultiplexer element 90-3 being connected to the cross output port OUT2b of the two-stage optical demultiplexer element 90-2 in the 1st stage. In a conventional optical demultiplexer, the output ports are selected so that the 1st stage through output port and the 2nd stage through output port are combined together and the 1st stage cross output port and the 2nd stage cross output port are combined together.
The output ports are thus selected because group delays in the 1st and 2nd stages offset each other to obtain zero group delay characteristics. Furthermore, even with a fabrication error in the single optical multiplexer or demultiplexer, the offset of the group delays serves to maintain substantially zero group delays. Thus, the conventional optical multiplexer or demultiplexer is characterized in that the group delay characteristics are unlikely to be affected by fabrication errors in the circuit.
However, although the group delay characteristics of the conventional optical multiplexer or demultiplexer is unlikely to be affected by fabrication errors in the circuit and are thus substantially zero, its transmission characteristics are prone to be affected by fabrication errors. A problem of the conventional optical multiplexer or demultiplexer is that a good stopband value is not obtained in the presence of a fabrication error.
When the optical multiplexer or demultiplexer is actually used, group delays have only to be maintained at a certain allowable value or less, and the optical multiplexer or demultiplexer element often needs to have as good transmission characteristics as possible.
It is an object of the present invention to provide an optical demultiplexer and an optical multiplexer which have transmission characteristics unlikely to be affected by fabrication errors and which have small group delay dispersions.
To attain this object, an optical demultiplexer comprising 2-input and 2-output (hereinafter referred to as the xe2x80x9c2xc3x972xe2x80x9d) optical demultiplexer elements composed of optical couplers that couple two optical waveguides together at N+1 locations (N is an integer equal to or larger than 2), each the 2xc3x972 optical demultiplexer element having a through output port and a cross output port for a single input port, in which a through output port of a first 2xc3x972 optical demultiplexer element is connected to an input port of a second 2xc3x972 optical demultiplexer element and a cross output port of the first 2xc3x972 optical demultiplexer element is connected to an input port of a third 2xc3x972 optical demultiplexer element, so that if a wavelength multiplex signal of a specified wavelength interval of xcex1, xcex2, . . . , xcexM (M is an integer equal to or larger than 2) is input to an input port of the first 2xc3x972 optical demultiplexer element, a wavelength multiplex signal of xcex1, xcex3, . . . , xcexMxe2x88x921 is output from a selected output port of the second 2xc3x972 optical demultiplexer element, while a wavelength multiplex signal of xcex2, xcex4, . . . , xcexM is output from a selected output port of the third 2xc3x972 optical demultiplexer element, wherein a cross output port of the second 2xc3x972 optical demultiplexer element is selected, and a through output port of the third 2xc3x972 optical demultiplexer element is selected, wherein the through output port of the first 2xc3x972 optical demultiplexer element has a passband equal to that of the cross output port of the second 2xc3x972 optical demultiplexer element, and the cross output port of the first 2xc3x972 optical demultiplexer element has a passband equal to that of the through output port of the third 2xc3x972 optical demultiplexer element, and wherein the through output port of the first 2xc3x972 optical demultiplexer element has group delay characteristics opposite to those of the cross output port of the second 2xc3x972 optical demultiplexer element, and the cross output port of the first 2xc3x972 optical demultiplexer element has group delay characteristics opposite to those the through output port of the third 2xc3x972 optical demultiplexer element.
According to this configuration, selection of the output ports eliminates differences in characteristics between the input port and the two output ports which differences result from a fabrication error in the circuit, thereby minimizing fabrication errors in transmission characteristics and maintaining group delay dispersions at an allowable value or less.
Further, an optical multiplexer comprising 2-input and 2-output (hereinafter referred to as the xe2x80x9c2xc3x972 xe2x80x9d) optical multiplexer elements composed of optical couplers that couple two optical waveguides together at N+1 locations (N is an integer equal to or larger than 2), each the 2xc3x972 optical multiplexer element having a through output port and a cross output port for a single input port, in which an output port of a first 2xc3x972 optical multiplexer element is connected to a through input port of a third 2xc3x972 optical multiplexer element and an output port of a second 2xc3x972 optical multiplexer element is connected to a cross input port of the third 2xc3x972 optical multiplexer element, so that if a wavelength multiplex signal of a specified wavelength interval of xcex1, xcex3, . . . , xcexMxe2x88x921 (M is an integer equal to or larger than 2) is input to a selected input port of the first 2xc3x972 optical multiplexer element, while a wavelength multiplex signal of a specified wavelength interval of xcex2, xcex4, . . . , xcexM is input to a selected input port of the second 2xc3x972 optical multiplexer element, then a wavelength multiplex signal of xcex1, xcex2, . . . , xcexM is output from the output port of the third 2xc3x972 optical multiplexer element, wherein a cross input port of the first 2xc3x972 optical multiplexer element is selected, and a through input port of the second 2xc3x972 optical multiplexer element is selected, wherein the through output port of the third 2xc3x972 optical multiplexer element has a passband equal to that of the cross output port of the first 2xc3x972 optical multiplexer element, and the cross output port of the third 2xc3x972 optical multiplexer element has a passband equal to that of the through output port of the second 2xc3x972 optical multiplexer element, and wherein the through output port of the third 2xc3x972 optical multiplexer element has group delay characteristics opposite to those of the cross output port of the first 2xc3x972 optical multiplexer element, and the cross output port of the third 2xc3x972 optical multiplexer element has group delay characteristics opposite to those the through output port of the second 2xc3x972 optical multiplexer element.
According to this configuration, selection of the input ports eliminates differences in characteristics between the two input ports and the output port which differences result from a fabrication error in the circuit, thereby minimizing fabrication errors in transmission characteristics and maintaining group delay dispersions at an allowable value or less.
According to the present invention, a certain level of fabrication error is allowed, thereby making it possible to produce an optical multiplexer or demultiplexer having high yield and mass productivity.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.